In telephone systems, line circuits serve as an interface between communication lines connected to subscriber sets and the telephone switching network. Traditionally, the line circuit has included a battery feed to energize the subscriber set. It has been recognized that the effect of longitudinal currents, which are frequently induced in a subscriber loop because of nearby electrical power lines, can be minimized when the batter feed is capable of floating with respect to ground. Known line circuits have implemented a floating battery feed by using a transformer to isolate the subscriber set from the office DC source.
In the floating battery feed circuit disclosed in U.S. Pat. No. 4,499,532 issued to R. C. Hudson et al., on Feb. 12, 1985, a switching-mode, flyback power converter uses a multi-winding transformer to supply power from a source of potential to an electrical load. A switching transistor is used to selectively connect the source to the transformer primary winding. When the transistor is turned on, energy from the source is stored by the transformer since a diode connected between the transformer secondary winding and the load is reverse biased to prevent secondary current flow. However, when the transistor is subsequently turned off, the voltages across the primary and secondary windings reverse polarity and the energy previously stored by the transformer is transferred to the load. As is typical of such power converters, the switching transistor is cycled at a relatively high rate, e.g., 64 kilohertz. A recognized problem in the art is that power converters of this type frequently generate unacceptable levels of high frequency, common-mode, electrical noise at their output terminals because of the high switching rate.